Use xvfb to run tests

.github/workflows/testing.yml

@@ -24,8 +24,8 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         if [ -f dev-requirements.txt ]; then pip install -r dev-requirements.txt; fi
-        # install PyQt5
-        python -m pip install PyQt5==5.12.3
+        # install PyQt5 & xvfb
+        sudo apt install -y python3-pyqt5 python3-pyqt5.qtsvg python3-pyqt5.qtwebsockets xvfb
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names
@@ -34,4 +34,4 @@ jobs:
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
-        pytest -vv
+        xvfb-run python3 -m pytest -vv